FR2586411A1 - METHOD AND DEVICE FOR REDUCING THE TOXICITY OF SULPHITES - Google Patents

Abstract

METHOD FOR REDUCING THE TOXICITY OF SULPHIDES CONTAINED IN WASTEWATER. THE WASTE WATER WITH A HIGH SULFUR CONTENT IS INTRODUCED IN A FIRST REACTOR IN WHICH THE PH IS KEEPED AT A VALUE LESS THAN 6; THE SULPHATES AND OR SULPHITES OF THE WASTE WATER ARE CONVERTED INTO SULPHIDE GAS BY MEANS OF A SULPHATE REDUCING BACTERIA; THEN THE SULFUROUS GAS IS REMOVED FROM THE REACTOR. THE DESULPHURATED WASTE WATER IS INTRODUCED IN A SECOND REACTOR TO CONVERT THE ORGANIC COMPOUNDS INTO METHANE GAS WHICH IS RECOVERED. APPLICATION TO WASTEWATER TREATMENT.

Description

25864 11

  METHOD AND DEVICE FOR REDUCING TOXICITY

SULPHITES

  The present invention relates to a method and a

  for the reduction of the sulfur content (sulphates or sulphites) of a sewage effluent to be treated anaerobically. The reduction of the sulfur content of the wastewater according to the present invention makes it possible to introduce into a reactor wastewater with a high sulfur content and to maintain the pH level in the reactor to a value of less than 6, the compounds sulphate and / or sulphite of waste water being converted

  in sour gas and removed from the reactor.

  Two-phase anaerobic methods are well known and described in U.S. Patents 4,067,801, 4,318,993 and 4,022. 665. The anaerobic process

  consists of using a biological process for the stabilization

  organic matter by conversion to methane gas in the absence of oxygen. The process generally involves using two groups of bacteria, acid-forming bacteria that convert the organic materials to volatile acids and then acetic acid and H2, and a second group of

  methane-forming bacteria that convert acetic acid

  tick, H2 and CO2 in methane gas. Although both groups of

  acid-forming and methane-forming bacteria may function

  simultaneously in a single reactor, it is preferable

  as described in the aforementioned patents, to have separate reactors for acid fermentation and

for methanogenesis.

  This two-stage anaerobic process for wastewater treatment is satisfactory in most cases. However, in cases where the waste water has a high sulphate content (SO 4 and sulphites (SO 3 =) as for wastewater from pulp processing, sulphates and sulphites

  are reduced to sulphides inside a reactor operating

  under anaerobic conditions. Sulphides, such as H2S and HS-, are known to be toxic to methanogenic (or methane-forming) bacteria even at a similarly low level.

  than 150 mg / l, and so many wastewater

  sulphates and / or sulphites can not be

or eliminated anaerobically.

  If sulphates and sulphites are present in wastewater at a concentration greater than 150 mg / l, they may be toxic to methanogenic bacteria, preventing or slowing down the conversion of acetic acid, H2 and C02. in methane gas. In this case, the use of an anaerobic system in the treatment of water containing

  Sulfur is quite impossible. The addition of metal salts

  However, such as ferric or ferric chloride in the reactor for precipitating metal sulphides has been used to reduce the sulphide concentration to allow anaerobic treatment of high sulfur wastewater as described in US Pat. United States of America N 4.318.993. However, the action of ferrous ions in the anaerobic process, and the cost of the necessary chemicals, have reduced the effectiveness of the anerobic process in the treatment

of this type of wastewater.

  The plaintiff has therefore discovered a method and an arrangement

  to reduce the toxicity of sulphides in the anaerobic treatment of high-sulfur wastewater, which is

  both economic and satisfactory in the treatment of

  erobie such wastewater. The present invention eliminates the problems and costs associated with reducing the sulfide contents of wastewater by the addition of metal salts or chemicals such as ferric or ferrous chlorides.

  to precipitate metal sulphides and eliminate

  precipitated sulphides. Other advantages of this

  invention will appear in the description below.

  The main purpose of the present invention is to propose

  a method and a device for reducing the toxicity of

  in high-sulfur wastewater. In part-

  The object of the present invention is to reduce the sulfur concentration of waste water to be anaerobically treated by introducing spent water into a reactor, maintaining the pH at a level below 6 in the reactor, and sulphate and / or sulphite conversion in this wastewater

  sulfurous gas, then expulsion of the sulfurous gas from the reactor.

  It is another object of the present invention to provide a sulphate reducing bacterium for converting sulphates.

and sulphites in sulfurous gas.

  The sulphate-reducing bacterium may be a bacterium

  selected from Desulfovibrio, Desulfotomaculum, Desulfobac-

tera and Desulfobulbus.

  Preferably, the invention provides for two aerobic reactors.

  series-connected bacteria, with sulphate-reducing bacteria in a first reactor to convert sulphate and sulphite from wastewater to sulphurous gas, and acid-forming and methanogenic-forming bacteria into a second reactor to convert organic products into gas methane, through organic acids. It is also an object of the invention that the acid-forming bacteria used to convert organic compounds to organic acids are contained in one or both reactors. It is also desirable, in accordance with the present invention, for the pH in the first reactor to be less than 6

  and that of the second reactor is about 7.

  The description, which will follow and which presents no

  limiting character, must be read with reference to the appended figures, among which: - Figure 1 is a block diagram of the treatment of wastewater according to the present invention;

  FIG. 2 is a block diagram concerning the use of

  fluidized bed reactor assembly as first and second reactor; and FIG. 3 is a graphical representation of the pH as a function of the percentage of dissolved sulphides present in

the reactor.

  The method and the device according to the present invention constitute a new means for reducing the sulfur content

  wastewater to be treated under anaerobic conditions.

  bies. This method and this device are preferably used when the sulphate or sulphite concentration of the wastewater is greater than 150 mg / l. Indeed, when the content

  sulphates or sulphites exceeds the concentration level

  previously, sulphides obtained by conversion of sulphates and / or sulphites have become toxic.

  against methanogenic bacteria and reduce substantially

  the rate of conversion of organic matter into

  methane. The present invention preferably comprises the use of

  two anerobic reactors, these reactors being fluidized bed reactors, mounted in series, with

  sulphate-reducing bacteria (SRB) -in the first reaction

  to convert sulphates and / or sulphites from waste water to sulphurous gas, and acid and methane forming bacteria mainly disposed in the second reactor to convert organic compounds to methane gas via organic acids. Most of the sulfides produced in the first reactor are in gaseous form (H2S) and can be easily removed from the first reactor, so that the wastewater is no longer toxic to the methanogenic bacteria in the second reactor. For this purpose, it has been found that certain conditions must be respected inside the first reactor, namely that the pH must be less than 6, so that more than 90% of the sulphides are H 2 S (gaseous) instead of HS (soluble). Maintaining the pH in the first reactor to a value below 6 is also favorable to the acid-forming bacteria that convert the compounds

organic to organic acids.

  The second reactor should be adjusted to a pH as close as possible to 7 in order to promote the growth of methanogenic bacteria. If the pH of the first reactor exceeds 6, the conditions prevailing in the reactor will allow the sulfides to remain in the wastewater and this wastewater will be transferred to the second reactor and may be toxic to methanogenic bacteria. SRB exists in all anaerobic natural environments where sulphates are available, such as sewage containing sulphates. Examples of SRB are species such as Desulfovibrio, Desulfotomaculum, Desulfobactera and Desulfobulbus. The Desulfovibrio species, particularly Desulfovibrio desulphuricans, is most commonly found in wastewater. These bacteria are curved rods, mesophilic, strictly

  anaerobes, with a single polar flagellum.

  Growth promoting media for SRB are also favorable for acid-forming bacteria. Thus, it can also be expected that the acid-forming bacteria will proliferate in the first reactor provided that the sulphide concentration is not too high and becomes toxic to the acid formers. The carbon needs of SRB can be covered by volatile acids produced by acid-forming bacteria, although other carbon sources, such as lactic, malic,

  and pyruvic, are preferred by SRB.

  The use of acetic acid during sulfate reduction is illustrated by the following biochemical stoichiometric equation:

CH3COOH + SO4 = SRB H2S + 2HCO3

  The gas produced in the first reactor is essential

  consisting of H2S and CO2, the secondary products of the

  sulphate reduction and the formation of acids.H2 can also

  be present when CH4 does not exist in the gas or vice versa, since H2 and CO2 are immediately reduced to CH4 in the presence of methanogens. The gas from the first reactor can be combined with the gas produced by the second reactor, which is supposed to contain a high proportion of CH4, to be reused if the proportion of H2S is low. However, if the gaseous H2S removed from the first reactor is present in high proportions, it is possible to carry out a downstream treatment of the gaseous H 2 S to recover the sulfur either as elemental S, or in the form of sulfuric acid, by chemical or biological route. Oxidation processes of

H2S currently exist.

  The present invention is not limited to a specific anaerobic method. The first and second reactors shown in FIG. 1 may be constituted by any of the following reactors: anaerobic contact reactors, anaerobic upflow or downflow filters, anaerobic sludge discharge reactors, beds

  anaerobic fluidizers or a combination of these reactors.

  However, the capacity of the process can be improved by using high speed processes such as beds

  fluidized anaerobes, shown in Figure 2.

  Figure 1 shows a two-stage anaerobic reactor process for the reduction of sulphide toxicity in accordance with the present invention. Wastewater with high sulfur content (sulphates and / or sulphites) is introduced into

  the first reactor for desulphurization.

  In the first reactor of Figure 1, the following reaction occurs: (1) SO4 + 2C (carbon source) + 2H20 SRB 2HCO + H2S (gas) <) 04 = 2 3 2S gz

  The method and device shown in FIG.

  to reduce the toxicity of sulphides, are

  Particularly suitable for the treatment of waste water from the treatment of pulp and paper which contains

  sulphates and / or sulphites in a markedly

  less than 150 mg / l. By adjusting the pH of the first reactor to less than 6, not only the reduction of sulphates or sulphites but also sulphides in gaseous form (H2S instead of HS) will be achieved, and thus the gaseous sulphide can be easily removed. of the reactor at the same time

  acidification products such as CO2 and H2.

  The desulfurized wastewater is then fed from the first reactor to the second reactor, as shown in FIG. 1, where the organic compounds of the desulfurized wastewater are converted by the methanogenic bacteria into methane, via organic acids. Then the methane gas and CO2 are removed from the second reactor and the methane gas

  produced during the anaerobic process is purified thereafter.

  The liquid effluent is discharged from the second reactor for one

  downstream treatment or immediate disposal in the envi-

  ronment. The gases produced in the first and second reactors can be treated as follows: the high H2S gas can be sent to a downstream processing unit for sulfur recovery, and the low H2S gas can be combined with the gas produced in the

  second reactor to be used later.

  Figure 2 depicts the use of fluidized bed reactors as first and second reactors in a method and apparatus for the reduction of sulfide toxicity, in accordance with the present invention. Figure 2 also shows the recycling of effluents from the first and second reactors. FIG. 3 represents the percentage of dissolved sulphides present in the form of H 2 S in relation to the percentage of dissolved sulphides present in the form of HS, as a function of the pH prevailing in the reactor. When the pH is about 5, about 100% of the sulfide produced in the first reactor is in the form of H2S - (gas) and can be easily

  eliminated from the device with minimal HS training.

  However, when the pH becomes greater than 6, the proportion of dissolved sulphides present in the form of H2S and HS begins to change rapidly. It is preferable that all sulphides in the wastewater are converted to H2S

  gaseous, to allow easy removal of the first reaction

  because the sulphides present as HS- are transported with the waste water to the second reactor, where they

  will be toxic to methanogenic bacteria.

  The present invention is not limited to the specific examples shown and the use of many types of reactors and bacteria in the same

  conditions than those described above.

Claims (12)

  1 - Process for reducing the toxicity of sulphides in wastewater, characterized by the fact that high sulfur waste water is introduced into a reactor, that the pH in said reactor is maintained at a temperature of value less than 6 and converting sulphates and / or sulphites from water   spent in sulphurous gas, after which the sulphurous gas is removed said reactor.   2 - Process according to claim 1, characterized in that the means for converting sulphates and sulphites   is a sulphate reducing bacteria.   3 - Process according to claim 2, characterized in that the sulphate reducing bacterium is selected from Desulfovibrio, Desulfotomaculum, Desulfobactera and Desulfobulbus.   4 - Method according to one of claims 1 to 3,   characterized in that the pH is maintained preferably between 5 and 6.   - Process according to claim 1, characterized in that the reactor is selected from anaerobic contact reactors, upflow anaerobic filters, anaerobic downflow filters, anaerobic slurry backflow reactors and fluidized beds. anaerobes. 6 - Device for reducing the toxicity of sulphides in wastewater, characterized in that it comprises a reactor receiving waste water with a high sulfur content, the pH prevailing in this reactor being less than 6; means for converting sulfates and / or sulfites from the waste water into sulfurous gas; and means for eliminating gases sulfurous reactor.   7 - Device according to claim 6, characterized in that the means for converting sulfates and sulfites   is a sulphate reducing bacteria.   8 - Device according to claim 7, characterized in that the sulphate reducing bacterium is   selected from Desulfovibrio, Desulfotomaculum, Desulfo bactera and Desulfobulbus.   9 - Device according to one of claims 6 to 8,   characterized in that the pH is maintained preferably between 5 and 6.   - Device according to one of claims 6 to 9   characterized in that the reactor is selected from anaerobic contact reactors, anaerobic   current, anaerobic low-current filters   the anaerobic sludge backflow reactors and anaerobic fluidized beds.   11 - Process for reducing the toxicity of sulphides by a two-stage anaerobic treatment of high-sulfur waste water, characterized in that waste water is introduced into a first reactor; maintaining the pH in the first reactor at less than 6; converting the sulphates and / or sulphites of the waste water to sulphurous gas, and then removing the sulphurous gas from the first reactor; and that the desulphurized wastewater is introduced into a second reactor to convert the compounds   organic wastewater first desulphurized with organic acids   and then methane gas and then recovering said gas.   12 - Process according to claim 11, characterized in that the means for converting sulphates and sulphites   is a sulphate reducing bacteria.   13 - Process according to claim 12, characterized by   the sulphate-reducing bacteria is selected   from Desulfovibrio, Desulfotomaculum, Desulfobactera and Desulfobulbus.   14 - Method according to one of claims 11 to 13,   characterized in that the pH is maintained preferably between 5 and 6.   - Method according to one of claims 11 to 14,   characterized in that the first and second reactors are selected from anaerobic contact reactors, upflow anaerobic filters, anaerobic downflow filters, anaerobic slurry backflow reactors, anaerobic fluidized beds and all   combinations of the reactors mentioned above.   16 - Method according to one of claims 11 to 15,   characterized in that the means for converting the organic compounds to acid is an acid-forming bacterium and the means for converting the organic acids   Methane gas is a methane-forming bacteria.